Update GraphQL mutation to support multi-level permissions

Created by: unknwon

Context: https://github.com/sourcegraph/sourcegraph/pull/10774#discussion_r427416571

This GraphQL mutation says it "sets...a full set", implying it overwrites all previous permissions for the repository. That makes sense and feels right.

However, what about when we support multiple permission levels (not just READ)? Then this no longer makes sense (because if you wanted to set some users as READ and some as ADMIN, then the 2nd operation to set ADMIN users would clear the READ users). I think we would need the API to be something like:

type Mutation {
  setRepositoryPermissionsForUsers(
    repository: ID!
    userPermissions: [UserPermission!]!
  )
}

input UserPermission {
  user: ID!
  permission: RepositoryPermission!
}

This might not seem like a problem now because we only support READ, but it will create confusion and extra work for our customers, so I think we should fix it ASAP.